(1) Field of the Invention
The present invention generally relates to an electric brake device, and more particularly to an electric brake device in which a brake motor is electrically driven in a forward direction so as to exert a braking force on a vehicular wheel, and the brake motor is electrically driven in a reverse direction so as to cancel the braking force on the vehicular wheel.
(2) Description of the Related Art
As disclosed in Japanese Laid-Open Patent Application No.7-291120, an electric brake device is known. The brake device of the above publication includes a brake motor, a disc rotor, and a pair of friction pads. The disc rotor is attached to a vehicular wheel, and the disc rotor is rotatable together with the vehicular wheel. The disc rotor is interposed between the friction pads. The disc rotor is pinched by the friction pads by rotation of the brake motor so that a braking force is exerted on the vehicular wheel via the disc rotor.
In the electric brake device of the above publication, the brake motor is connected through a mechanical clutch to a brake caliper. In the brake caliper, an internal cylinder is formed and a piston which is slidable on the cylinder is provided in the brake caliper. The piston is movably supported on a reversible screw shaft. The piston is secured to one of the friction pads. A movement of the piston in the axial direction of the cylinder, which causes the braking force of the friction pads to be exerted on the disc rotor, is derived from a rotation of the reversible screw shaft.
An output shaft of the brake motor is connected to the reversible screw shaft via the mechanical clutch. When a brake pedal is depressed by a vehicle operator, the brake motor rotates in the forward direction by a power supplied to the brake motor in response to the brake pedal depression. As the reversible screw shaft is rotated in the forward direction by the brake motor, the piston is moved to push the friction pads on the disc rotor. By the friction between the friction pads and the disc rotor, the braking force is exerted on the vehicular wheel. When the brake pedal is released from the brake pedal, the brake motor rotates in the reverse direction so as to separate the friction pads from the disc rotor. The mechanical clutch acts to cut off the transmission of the torque from the brake motor to the reversible screw shaft. By the separation of the friction pads from the disc rotor, the braking force on the vehicular wheel is canceled.
In the electric brake device of the above publication, by electrically driving the brake motor in a controlled manner, the electric brake device can generate an appropriate braking force to be exerted on the vehicular wheel.
However, there may be a case in which the output shaft of the brake motor is subjected to excessive reverse rotation when the brake motor rotates in the reverse direction. In such a condition, the excessive reverse rotation of the brake motor is detrimental to the related parts which are connected to the output shaft of the brake motor. It is necessary to reliably prevent the excessive reverse rotation of the brake motor in order to avoid the failure of the related parts connected to the output shaft of the brake motor.
In the electric brake device of the above publication, the mechanical clutch is provided to cut off the transmission of the torque from the brake motor to the reversible screw shaft. When the reversible screw shaft is separated from the brake motor by the mechanical clutch, the reversible screw shaft is not rotated regardless of whether the brake motor excessively rotates in the reverse direction. The mechanical clutch enables the electric brake device of the above publication to prevent the excessive reverse rotation of the brake motor so as to avoid the failure of the related parts connected to the output shaft of the brake motor.
However, the electric brake device of the above publication requires a complicated structure and a relatively high cost due to the use of the mechanical clutch. According to the above publication, it is difficult to construct a simple, inexpensive electric brake device which is reliable in preventing the excessive reverse rotation of the brake motor.